Forty-five years ago today (July 20), humanity took a giant leap from its birthplace onto the surface of another world.

On July 20, 1969, Neil Armstrong clambered down the ladder of the Apollo 11 lunar lander and pressed his boot into the moon's gray dirt — a simple if clunky step, witnessed by billions from afar, that stands as perhaps the most memorable moment in all of human history.

"The whole world stopped to watch what was taking place," space historian Roger Launius, assistant director of collections and curatorial affairs at the Smithsonian's National Air and Space Museum, told Space.com. "I think it would not be an overstatement to say that." [Apollo 11's 45th Anniversary: Complete Coverage]

Armstrong and his Apollo 11 crewmates, Buzz Aldrin and Michael Collins, launched from Florida's Cape Kennedy (now known as Cape Canaveral) on July 16, 1969. The mission aimed to achieve a goal President John F. Kennedy laid out for NASA in 1961 — to, by the end of the decade, land people on the moon and bring them back safely to Earth.

The astronauts reached their destination four days after liftoff. Armstrong and Aldrin descended to the surface in their lander, known as Eagle, while Collins remained in lunar orbit in Apollo 11's command module, which was named Columbia.

Armstrong, Apollo 11's commander, went first, uttering one of history's most famous sentences when his foot touched down: "That's one small step for [a] man, one giant leap for mankind."

While Armstrong found the surface of the moon beautiful, the stark and austere landscape struck his fellow moonwalker a bit differently.

"This is the most desolate thing I've ever seen," Aldrin told Space.com recently during an Apollo 11 anniversary chat, recalling what he was thinking as he ambled about the lunar surface 45 years ago. (You can watch a video of our conversation with Buzz Aldrin here.) "Not beautiful."

Armstrong and Aldrin stayed on the moon for about 21.5 hours, then blasted off to rejoin Collins in the command module. The trio splashed down safely in the Pacific Ocean on July 24, 1969, bringing their historic mission to a successful end.

The astronauts were initially placed into quarantine upon their return home, as scientists and doctors were concerned about the possibility of moon microbes being introduced to Earth. The Apollo 11 crewmembers left quarantine on Aug. 10 of that year, embarking on a sustained victory lap that included parades, speeches to Congress and meetings with world leaders around the globe.

A product of the Cold War

The scope and ambition of NASA's Apollo program seem staggering today. President Kennedy announced his bold goal just four years into the Space Age; at the time, the United States had not even launched a human into orbit yet. But just eight years later, Armstrong and Aldrin were walking on the moon. [NASA's 17 Apollo Moon Missions in Pictures]

This stunning achievement required the investment of about $25 billion, experts say — well over $100 billion in today's dollars. (During the height of the Apollo program, NASA's share of the federal budget was about 4 percent. Today, that figure hovers around 0.5 percent.)

The United States' headlong rush to the moon can only be understood within a Cold War context, Launius and other historians stress. The Soviet Union's two key space-race successes — launching the first satellite, Sputnik 1, in 1957, and putting the first person into orbit, in 1961 — spooked President Kennedy and other U.S. leaders. They saw in these achievements a grave threat to American technological superiority, and to national and global security.

Apollo was viewed as a way to help set things right, from an American perspective.

"Apollo was established as a program for a specific political purpose, and it achieved that purpose in a stellar way," Launius said. "It totally beat the Russians, which is what the objective was. It demonstrated to the world, as it was intended to do, that the United States was second to none when it came to technology."

This display of technological might came as the two superpowers were fighting for global influence — no mere coincidence, Launius added.

"That was one of the objectives — to make sure those nonaligned nations, especially those newly independent nations after World War II, throw their lot in with the Americans," he said. "That was a very successful objective."

NASA made five more manned moon landings after Apollo 11, with the last one reaching the lunar surface in December 1972. Nobody has set foot on the moon since.

A future on Asteroids and Mars

NASA's plans for the future of human spaceflight extend beyond the moon.

For example, the agency aims to grab a near-Earth asteroid and drag it into a stable orbit around the moon using a robotic probe. Once the rock is in orbit, astronauts could repeatedly visit it for scientific and exploration purposes. (The first manned trip is slated to occur by 2025.)

The agency views this bold asteroid-capture mission as a stepping stone toward the ultimate goal: Mars. NASA is committed to getting people to the vicinity of the Red Planet by the mid-2030s, as directed by President Barack Obama in 2010.

As NASA works to make these ambitious goals a reality, the agency continues to draw inspiration from the success of Apollo 11, officials said.

"I feel a very special obligation to say, 'Thank you, Neil, Buzz and Mike,'" NASA Administrator Charles Bolden said in a NASA video commemorating the 45th anniversary of Apollo 11. "We're standing on your shoulders, building on your historic achievements and getting ready to take the next giant leap for humankind."

Mike Wall

Michael was a science writer for the Idaho National Laboratory and has been an intern at Wired.com, The Salinas Californian newspaper, and the SLAC National Accelerator Laboratory. He has also worked as a herpetologist and wildlife biologist. He has a Ph.D. in evolutionary biology from the University of Sydney, Australia, a bachelor's degree from the University of Arizona, and a graduate certificate in science writing from the University of California, Santa Cruz. To find out what his latest project is, you can follow Mike on Google+.